The global burden of breast cancer is on the rise making it the malignancy with the highest incidence among women worldwide. Yet, much of the international variation of breast cancer incidence cannot be explained with known or suspected risk factors. Attention has shifted to earlier time periods of a women's life recognizing the specil susceptibility of the prepubertal mammary gland tissue to environmental influences. The considerable time lag between events prior to or at puberty and diagnosis of breast cancer spanning several decades have made prospective studies difficult to accomplish. Early onset of mammary gland development and high breast density have been associated with an increased risk of breast cancer and can serve as early putative markers of breast cancer risk. A high proportion of radiographically dense area of the breast is one of the strongest known risk factor for breast cancer and may be established at the time of breast development at puberty. We propose to use an existing cohort of about 515 prepubertal Chilean girls born in 2002 to address the relation between putative early markers of breast cancer risk, specifically onset of mammary gland development (Tanner stage 2) and volume of dense breast tissue measured by DXA at Tanner stage 4 and a number of antecedents: endocrine-disrupting chemicals, specifically bisphenol A and phthalates; frequent consumption of high fat dairy products and red meat; blood levels of inflammatory markers, specifically interleukin-6, tumor necrosis factor, and C-reactive protein; global DNA methylation, promoter methylation pattern, and methylation of the ER gene. PUBLIC HEALTH RELEVANCE: Assessing antecedents of the onset of mammary gland development and of breast density at puberty is of paramount importance for understanding determinants of breast cancer susceptibility and pathways through which early breast development and breast density affect breast cancer risk.